Pumps with servo-type actuation for cheek plate unloading

ABSTRACT

The pressure acting on a pressure loaded cheek plate in a pump is selectively vented by a servo valve actuated by the buildup of pressure in the pump outlet caused by normal parasitic losses in a downstream hydraulic flow circuit. Since it may be desirable to actuate the servo valve by the pressure drop across an orifice in the outlet line or by a suitable orifice in the pump return to pump, both embodiments are disclosed as exemplary means for achieving the premises of this design.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to the control of flow and pressurerelief functions in a pump and more particularly relates to a pumphaving an integral flow control arrangement incorporated therein.

2. Description of the Prior Art

The prior art is replete with pumps having integral flow control valves.However, most of these require the valve structure to by-pass the fullpump output to inlet. Sometimes such a flow control valve is locatedexteriorly of the pump in a conduit leading to the point of utilization.In other forms of pump construction, the flow control valve is made apart of the pump package by being incorporated directly in passages of apump housing, or in still other instances as an integral part of variouscomponents combined in a pumping cartridge. In either case, internalvalve by-pass at top speeds may result in 30 to 40 gpm flowing throughthe valve with resulting high Bernoulli reaction forces developing.These forces require high counter spring forces to close the portswithout sticking or valve hysteresis. Added spring force results insignificant increases in flow orifice pressure drop and its outwardtoken, heat rise; the latter resulting in significant cost imposts foroil coolers to secure safe heat levels, that will prevent component wearand hose failures.

Many means for limiting the tendency toward end seizure have been usedin the art. Fixed end clearances carefully monitored by quality controlmethods are difficult to control and represent the largest single endsealing means. Flexible end plates made of bronze or other premiummaterials have also been successfully applied at cost premium to variousdesigns. Neither alternative is totally ideal.

SUMMARY OF THE INVENTION

In accordance with the principles of the present invention, a completelydifferent operating concept is favorably exploited. Thus, in accordancewith the method and structure of the present invention a cheek platewith a sealing surface for establishing a seal between the inlet andoutlet portions of the pump is pressure-loaded with fluid atpump-generated pressure.

In a first embodiment fluid discharged by the pump may be directed topass through an orifice before proceeding to the gear or other motoringmeans.

A second embodiment directs the discharge to a point of utilization, andthe spent fluid is returned through an orifice on the return line sideof the pump.

In both cases, the resulting orifice drops are used to actuate a servovalve bias position in accordance with the changing pressure dropsthrough an orifice with varying pump speed. The biasing of the servovalve opens a modulated passage area opening from the static chamber tothe rear of the cheek plate to selectively vent this cavity. Since thepressure force at the rear of the cheek plate drops in accordance withthe valve bias set by a servo valve spring and an orifice, flow controlis achieved by the lifting of the cheek plate from the abutting camsealing surface and the direct by-pass from outlet to inlet of the pumpoutput above the flow control set point. As a result, flow control canbe accomplished with a small servo valve which establishes together withthe movable cheek plate a relationship similar to a servo ratio effect.In other words, a servo control system is provided wherein the servovalve operates as the master and the cheek plate operates as the slave.It is also clear, the absence of the conventional restricted egress andingress through outlet or into inlet which is characteristic in allspecies of vane or gear pump designs, is effectively overcome by theunloading plate's lift, resulting in a superior pump internal pressurecontrol, lower mechanical torque, and the lessening of cavitationaltendencies. Supercharge is achievable in the "pressure orifice unloadingplate design" by providing a series of nominally sized holes on theperiphery of the shell connecting the reservoir to the interior pumpplenum chamber at a point adjacent to the point where the rapidlyexpanding high velocity stream from the unloading plate by-pass isejected.

Still another option is obtained by using the return line flow toaspirate the reservoir oil. The latter arrangement may be used tosupercharge the return line orifice circuit in a similar manner.

The flow control concepts thus provided in accordance with theprinciples of the present invention have other important benefits. Avalve can be provided which is either part of the same physicalstructure or separated. Still another salient point is that a trigger orfull flow relief can be provided in either of the two embodimentsdescribed.

The lifting of the cheek plate above flow control or at pressure reliefconstitutes still another formidable advantage in the production,efficiency, and durability of the unloading plate concept.

Productively, the manufacturer may elect to build pumps with broader orcloser end clearance ranges thereby availing himself of freedom from thethreat of seizure. Moreover, efficiency rises as lower, closer endclearances are achievable. Durability and reliability are enhancedthrough the virtual elimination of end clearance seizure, i.e., theunloaded plate maintains an oil film surface with its adjacentcounterpart, the rotor or gear.

It will be apparent to those skilled in the art that the unloading platedesign is applicable to all rotating pumping types where the featuresascribed above fall within the field of its operating matrix.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a pump incorporating the principlesof the present invention;

FIG. 2 is a cross-sectional view taken on line II--II of FIG. 1;

FIG. 3 is an enlarged fragmentary cross-sectional view showingadditional details of the supercharging feature of the presentinvention;

FIG. 4 illustrates one arrangement of pressure relief available inaccordance with the teachings of the present invention;

FIG. 5 is a fragmentary cross-sectional view illustrating additionaldetails of the egress path provided in the pump of FIG. 1;

FIG. 6 is a cross-sectional view generally similar to the arrangement ofFIG. 1 but showing an alternative pump construction embodying theprinciples of the present invention and showing a reservoir surroundingthe pump;

FIG. 7 is a cross-sectional view taken on line VII--VII of FIG. 6;

FIG. 8 is a cross-sectional view taken on line VIII--VIII of FIG. 7;

FIG. 9 is a cross-sectional view of an alternative pressure relief;

FIG. 10 is a fragmentary cross-sectional view somewhat similar to FIG. 1but showing an alternative form of supercharging; and

FIG. 11 is a cross-sectional view showing a pressure orifice circuitutilizing a servo valve in accordance with this invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

While the principles of the present invention are described herein inconnection with a specific configuration of pump, namely, a pump of thetype utilizing as pumping elements slippers which are free to move.[.radialy.]. .Iadd.radially .Iaddend.and to rock angularly in followingthe adjacent bore wall contour, it should be understood that theprinciples of the present invention are applicable to any form of pumpwherein a pumping chamber is provided with inlet and outlet portions andan adjoining cheek plate is pressure-loaded to seal such inlet andoutlet portions from one another during normal pumping operations. Forexample, such pumps could include gear pumps, vane pumps, and othergeneral forms of rotary fluid displacement means.

In FIG. 1, there is shown a pump indicated generally at 10 andcomprising a housing 11 providing a flat end surface 12. A drawn steelshell 13 is of generally cup-shaped configuration and has an end wall 14and side walls 16 terminating in an end flange 17 provided with aninternal recess 18. The shell 13 fits over the housing 11 and a lockingring 19 seated in the groove 18 engages the housing as at 20, thereby tolock the same in assembly with one another.

Interiorly of the steel shell 13 there is provided a cartridge packwhich constitutes the various elements or components forming a pumpingunit. Thus, there is a lower pressure plate Lp disposed adjacent thesurface 12. The components of the cartridge pack are arranged in anaxial row and the next element is a cam ring Cr in which is formed apumping chamber. A shaft 21 which may be rotated from a suitable powersource, extends into the housing 11 and has a spline portion 22interfitting with a correspondingly internally spline portion 23 of arotor 24. The rotor has a plurality of circumferentially spaced notchesand in each notch there is disposed a slipper 25 which operates to movefluid from an inlet portion of the pumping chamber to an outlet portionof the pumping chamber at increased pressure. As noted, the pump of thepresent invention may conveniently comprise pumping elements which areslippers of the type movable radially and rockable angularly as theyfollow the adjoining bore wall of the pumping chamber.

An upper pressure plate Up is provided which has a sealing face 30 onone side thereof adapted to engage the cam ring Cr and operating to sealthe inlet portions of the pump from the outlet portions of the pumpduring normal pump operations.

The upper pressure plate Up is circumferentially notched as at 31thereby to seat an O-ring sealing member 32, which sealing member 32also engages the adjoining wall 16 of the shell 13, thereby to sealinglyseparate the inlet portions of the pump designated at 33 from the outletcavity of the pump shown at C2.

A backing plate 34 engages against a rear surface of the upper plate Upwhich is shown at 36 and which constitutes a motive surface extendingacross the rear of the upper plate Up and thereby constituting one wallof the cavity C2 which, in effect, forms a pressure control chamber.

The various components of the cartridge pack which are disposed in anaxial row are initially spring-biased by a coil spring 37 bottomed atone end against the wall 14 of the shell 13 and bottomed at its oppositeend against the backing plate 34. Thus, all of the elements are heldtogether upon pump start-up. As soon as the pump is in operation, fluidat pump-generated pressure is directed through a pump outlet port 38through an orifice O4 into the cavity C2.

In accordance with the principles of the present invention, the elementsof the cartridge pack are retained in alignment by means of dowel pinsof special construction. Thus, it will be noted there is provided adowel pin 40 which constitutes a hollow tube, thereby to provide anaxially extending passage P3 through the interior thereof. A ball 41seals the open end of the dowel pin 40, but there is formed an orificeO2 in the wall of the dowel pin, thereby communicating the passage P3with the cavity C2.

Another dowel pin is shown at 42 and has a hollow tubular constructionthereby to form an internal passage designated at P4. A ball 43 sealsthe end of the dowel pin 42, but there is also formed an orifice O3which communicates the passage P4 with the cavity C2.

As shown in FIG. 5, the pump output is directed from the outlet portkidneys formed in the upper plate Up and designated at OPK through apassage hole formed in the cam ring Cr designated P6 through a passageP7 formed in the lower plate Lp and into a passage P8 of the housing 11and into a cavity C4 which is in communication with an appropriateconnecting outlet line adapted to be threaded into a fitting F of thehousing 11. The fluid at pump generated pressure is thus directed to apoint of utilization. For example, if the pump shown in the drawings isutilized as a power steering pump, the fluid is then directed to thepower assisting device. After the fluid is spent and is returned, thereturn fluid is directed into a cavity C1 formed in the housing 11 inthe bottom of a return line connection shown at 50 (FIG. 1). The returnline flow entering the cavity C1 passes into a passage P1 formed in thehousing 11 and thence passes through an orifice O1 formed in an exteriortube T1 as shown in the enlarged detail of FIG. 3. The tube T1 hasconcentrically telescoped therewithin a second tube T2 which is lancedas at 51 to form an opening 52 which is in register with the orifice O1.The lance 51 overlies the opening 52 and is disposed in front of theorifice O1, thereby to maintain laminar flow from the orifice O1 intothe interior of the tube T2 so that a supercharge flow effect isgenerated by the orifice flow. Such flow effect induces flow through thetube T2 and a center passage P20 provided by the hollow interior of thetube T2 and which is connected to the reservoir 60. The flow momentum,resulting from this action, forces the supercharge flow via the passageP2 into the pump inlet Ip of the lower plate Lp.

With respect to the flow control action, it will be noted upon examiningthe drawings that the return line flow entering the cavity C1 in passingthrough the orifice O1 causes a pressure drop to register on theforepart of a flow control valve Vfc, which control surface is indicatedat S1. The valve Vfc under the force of the flow drop, moves to open thepressure control chamber or rear cavity C2 behind the pressurized upperplate Up through a passage P2 which is controlled by the flow controlvalve Vfc, the passage P2 being in fluid communication with the passageP3 and the hollow dowel pin 40 and being connected to the cavity C2 bythe orifice O2 and venting to a passage P11 which may be connected to azone at lower pressure, for example, the reservoir. Thus, pressure inthe cavity C2 drops and allows the upper pressure plate Up to move offof the cam surface provided by the cam ring Cr allowing outlet pressureto bypass directly to the inlet portions of the pump adjacent to thesealing face 30 of the upper pressure plate Up and also the adjoiningfaces of the rotor 24.

The flow control valve Vfc is initially spring biased with a controlloading spring shown at Sp1. That spring is set in place by a ballretainer 62 (FIG. 1). Accordingly, the flow control valve Vfc can bepreset with the use of small spring Sp1 and functions as a servo valveto regulate the pressure acting on the end plate or cheek plate formedby the upper pressure plate Up. It will be understood that the orificeO4 is smaller than both the orifice O2 and the orifice O3, therebyallowing the pressure in the cavity C2 to drop when the flow controlvalve Vfc is actuated. By virtue of such provision, the cheek plateoperates as the slave of the servo valve which functions as the masterin the servo control system.

In the form of the invention illustrated in FIG. 4, the pressure relieffunction occurs when the pressure in a cavity C4 which is connected tothe cavity C3 via the orifice O3 in the dowel pin 42 and the passage P4and passage P5 reaches relief pressure. A ball B moves off of a seatring Se thereby compressing a spring shown at Sp2. Since the cvity C3 isconnected to low pressure via a passage P9, the pressure level in thecavity C2 at the rear of the upper pressure plate Up is controlled at apressure level set by the tension in the spring Sp2 which is the reliefsetting.

Referring now to FIGS. 6, 7 and 8, the structure of the pump isdisclosed in somewhat more detailed fashion. However, since most of theelements correspond in structural configuration and function, likereference numerals will be used to identify like parts. Thus, there is apump shown generally at 10 comprising a housing 11 providing a flat endsurface 12. A drawn steel shell is again indicated generally at 13 andhas an end wall 14 and side walls 16 which terminate in an end flange 17forming an internal groove 18 in which is seated a locking ring 19engaging an adjoining portion of the housing 11.

Interiorly of the shell 13 and cooperating with the housing 11 is acartridge pack containing an axial row of pump components including alower pressure plate Lp, a cam ring Cr and an upper pressure plate whichis generally designated at Up but which may have bonded together partsforming a sealing face 30 and a backing plate 34 and a motive surface36. An O-ring 32 is received in the outer portions of the upper pressureplate Up in a recess 31, thereby to sealingly separate an inlet portion33 surrounding the cartridge pack and an outlet cavity C2 behind thepressure plate Up. A compression spring 37 is disposed between the wall14 and the upper pressure plate Up to provide initial cartridge loadingand biasing force. A stud 70 has a head 71 internally of the shell wall14 and a threaded portion 72 extends outwardly for mounting purposes. Areservoir shell 73 is apertured to fit over the mounting stud 72 and asealing gasket 74 is interposed between the reservoir shell 73 and alocking nut 76. The reservoir shell has a filling opening at 77 closedby a cap 78 and engages the housing 11 as at 79. An O-ring sealingmember 80 completes the seal, thereby permitting the entire interior ofthe shell 73 to form a reservoir previously referred to by the referencenumeral 60 (FIG. 3).

In the form of the invention illustrated in .[.IFG..]. .Iadd.FIG..Iaddend.6, the slippers are clearly shown at 25 and are biased radiallyoutwardly by loading springs 25a. Further, the shaft 21 is shownjournalled in a sleeve bearing 21a and a shaft seal is provided in thehousing 11 as shown at 21b.

In the form of the invention shown in FIG. 6, the hollow dowel pin isshown at 40a and opens directly into the cavity C2. Referring now toFIG. 8, the cavity C4 receives fluid from the pump outlet ports anddischarges the same through an opening 86 in a seat insert 87 located inthe threaded fitting F. It will be noted that a flow control valve 80 isspring-biased by a flow control valve spring 81. The valve is of agenerally tubular form spool but has a tapered surface 85. Moreover, thevalve 80 moves in a sleeve 82 which provides a cylinder having lands andrecesses for permitting the development of a valving action.Specifically, the sleeve 82 is provided with an orifice 83 whichcommunicates with the passage extending through the dowel pin 40a,namely, the passage P3. Flow from the pump return line enters a cavityC8 and drops across an orifice to produce a pressure drop which willmove the flow control valve 80. Because of the taper 81, the opening 83will be opened gradually thereby venting the cavity C2 via the dowelpassage P3. The tapered surface insures that the flow control valve 80moves a significantly greater distance than the pressure plate Up movesto maintain the flow control level. This provides greater stability inthe valving system and is somewhat similar to a servo ratio effect.

In the form of pressure relief shown in FIG. 6, such pressure relief isdependent upon the flow control valve. Flow enters the outlet cavity C4(FIG. 8) from the outlet port responding to a demand from the point ofutilization until it approaches relief pressure. At relief pressure, aball B2 biased by a spring B3 lifts allowing flow to pass to the cavityC8 and the orifice O5, thereby triggering the flow control valve 80 toopen in the manner described and by-pass directly across the sealingsurface 30.

To provide pressure relief function independent of flow control thearrangement of FIG. 9 may be provided wherein the pressure in cavity C4exceeds the setting of spring Sp2 moving the spool S2 and bringing aflute-like slot Sf into the cavity C5 which is at atmospheric pressure.The pressure level translated through the passage P4 from the cavity C2drops to maintain the pressure in the cavity C4 in accordance with thepressure setting of the spring Sp2 thereby regulating pressure relief.

Referring now to FIG. 10, a pump is shown at 90 having a shell 91 formedwith holes 92 formed in the outside diameter of the shell 91 outwardlycircumjacent the cheek plate 93. With this concept the holes 92 areconduits for flow of make-up fluid from the reservoir R enclosed betweenthe shell 91 and a cam 94. The first by-pass over the cam ring 96 fromthe plate lift inducts oil from the reservoir R to accomplishsupercharge.

In FIG. 11, a pressure orifice circuit arrangement is shown. A pumphousing 99 has a successively counterbored recess 100 terminating in anend wall 101 bottoming a relief valve assembly 102. A passage 103 in thehousing 100 returns flow from the relief valve to the reservoir. Asleeve 104 is sealed in the recess 100 by an O-ring sealing member 106and also forms a valve seat 107 for a valve ball 108 spring-pressedtoward closed position by a relief valve spring 109, the other end ofwhich is bottomed against a closure plug 110.

A control spool sleeve 111 is sealed in the recess 100 by threelongitudinally spaced O-ring sealing members 112, 113 and 114. One ormore openings 116 communicate the interior of the sleeve 111 to anannular servo by-pass leading to the reservoir and which annular by-passrecess is shown at 117 formed between the sealing members 112 and 113.

One or more openings 118 formed in the sleeve 111 between the sealingmembers 113 and 114 forms a valving recess controlled by a correspondingvalve land 119 formed on one end of a control spool 120 axially movableinside of the sleeve 111. The spool 120 has a hollow interior formedwith a shoulder 121 which bottoms one end of a coil spring 122, theother end being bottomed against the sleeve 104, thereby tending to loadthe control spool 120 in one direction. The valving recess and openings118 communicate with a passage 123 corresponding to the passage 40aalready described, thereby constituting a static chamber conduit leadingto a static chamber corresponding to C2.

The control spool 120 has a front land 124 slidably engaging the sleeve111 inwardly of the O-ring seal 112.

A seat insert 126 is formed with a passage 127 leading to a pump outlet128 and also has a throat 129 at one end forming a sized opening. Thecontrol spool 120 has a valve head 130 which seats against the edges ofthe throat 129 thereby closing the sized opening and has a stem 131projecting through the opening. The stem 131 is tapered as at 132 andhas a sense opening 133 formed in the end wall.

An annular space axially inwardly of the seat insert 126 is shown at 134and constitutes a pressure cavity corresponding to C4 receiving fluidfrom the pump outlet ports through a passage 136.

The control spool 120 operates as a servo valve and the stem 131 on thespool 120 serves two purposes. Firstly, it allows a sense to beestablished at the rear valve cavity without drilling difficult holepatterns to connect the fore and rear parts of the valve. Secondly, thestem 131 provides a means for setting up a tapered pin via the taperedsurface 132 through an orifice provided by the throat opening 129,thereby to achieve a drooping flow characteristic, if necessary ordesirable.

In operation, the pressure drops across the orifice 129 actuate theservo valve spool 120 to different bias positions in accordance with thevarying pump speed. The biasing of the servo valve opens a modulatedpassage 123 opening from the static chamber C2 to the rear of the cheekplate 93 to selectively vent such chamber. Since the pressure force atthe rear of the cheek plate drops in accordance with the valve bias setby the spring 122 and the orifice 129, flow control is achieved by thelifting of the cheek plate 93 from the abutting cam sealing surface ofthe cam ring 96 and the direct by-pass from outlet to inlet of the pumpoutput exceeding the flow control set point. Thus, flow control isachieved with a small servo valve which together with the movable cheekplate 93 establishes a relationship similar to a servo ratio effect.

Although various minor modifications might be suggested by those versedin the art, it should be understood that we wish to embody within thescope of the patent warranted hereon all such modifications asreasonably and properly come within the scope of our contribution to theart.

What we claim as our invention: .[.1. The method of flow control in arotary pump which comprises the steps of the pump flow output..]. .[.5.In a pump as defined in claim 4, said means forming an orifice formingsaid orifice in the return line of the pump and through which spentfluid is directed on the return line side of the pump..]. .[.6. In apump as defined in claim 4, said means forming an orifice forming saidorifice in the outlet line of the pump through which fluid discharged bythe pump is directed before proceeding to a motoring means..]. .[.7. Ina fluid pump as defined in claim 4,said servo valve comprising a spoolvalve having a tapered spool movable in a bore having lands andrecesses, spring means initially loading said spool in one direction,said spool controlling the venting passage by gradually opening thepassage due to the tapered configuration, thereby to produce an axialmovement of the cheek plate but through a significantly lesser distancethan the corresponding movement of the spool to maintain a flow levelcorresponding to a servo ratio effect in the flow regulation of thepump..]. .[.8. In a pump as defined in claim 4 and further characterizedby means forming a metering passage from a source of pump generatedpressure to said static cavity, and means forming a metering orifice forsaid metering passage..].
 9. In combination,a pump having housing meansforming a pumping chamber having inlet and outlet portions, rotary fluiddisplacement means in said pumping chamber for moving the fluid atincreased pressure from the inlet to the outlet portions, a cheek plateadjacent said pumping chamber having a sealing face for sealing theinlet and outlet portions during operation of the pump, means includingsaid housing forming a static pressure control chamber behind said cheekplate and having an orificed metering passage to load said cheek platewith fluid at pump-generated pressure, .Iadd. a seal between said cheekplate and said housing blocking flow into said cavity except throughsaid orificed metering passage, .Iaddend. means including said housingforming a pumping circuit including an outlet line and a return line forsaid pump receiving said spent fluid returned to the pump, means formingan orifice in said pumping circuit, and a servo valve controlling aventing passage in said housing venting said pressure control chamber toa zone at lower pressure and having actuating means responsive to thepressure drop across said orifice, thereby valving the cheek plate inservo ratio proportion with the servo valve and directly by-passing theoutlet portions of the pump to the inlet portions adjacent the sealingface of the cheek plate to modulate pump flow output.
 10. A pump asdefined in claim 9 wherein said orifice is formed in said return lineand through which spent fluid is directed on the return line side of thepump.
 11. A pump as defined in claim 9 wherein said orifice is formed inthe outlet line of the pump and through which fluid discharged by thepump is directed before proceeding to a point of utilization.
 12. Theinvention of claim 9 and further characterized bya hollow dowel pinextending through said cheek plate and into said housing means andforming a passage extending therethrough communicating at one end withsaid pressure control chamber, said servo valve controlling said passageat the other end of said passage in said dowel pin for selectivelyventing the pressure control chamber to a zone of reduced pressure,whereupon unloading of the cheek plate will by-pass the outlet portionto the inlet portions directly across the sealing face of the cheekplate.
 13. In a pump having an axial row of parts including a cam ringforming a pumping chamber and an end plate formed with a sealing faceadjacent the cam ring and means forming a pressure control chamberbehind the end plate, the improvement ofhollow dowel pin means extendingthrough the axial row of parts and having formed therein a ventingpassage communicating at one end with the pressure control chamber,orifice means for sensing a pressure gradient in the pump, and a servovalve in the pump responsive to changes in the pressure gradientcontrolling the other end of the venting passage to a zone of reducedpressure, thereby to selectively unload the end plate for by-passing theoutlet portions of the pump directly to the inlet portions of the pumpadjacent the sealing face.
 14. In a pump having a return line,a valve insaid return line having a tapered spool movable in a valve bore havinglands and recesses, spring means initially loading said spool in onedirection, means forming an orifice in said return line through whichfluid in the return line is directed, said spool being movable as afunction of the pressure drop across said orifice, and by-pass meanscomprising a pressure-loaded end plate having a sealing face forming onewall of a pumping chamber and having a motive surface loaded by fluid atpump-generated pressure, a venting passage for venting the pressureacting on said motive surface, said valve controlling said ventingpassage by gradually opening the passage due to the taperedsurface,thereby to trigger a movement of the end plate to directlyby-pass the outlet portions of the pump to the inlet portions thereof,said valve moving a significantly greater distance than said end plateto maintain a flow level,thereby to provide a servo ratio effect in theflow control regulation of said pump.
 15. In a pump,.Iadd.a housing,.Iaddend. a cam ring .Iadd.in said housing .Iaddend.forming a pumpingchamber for a fluid displacement means operable in said pumping chamber,a cheek plate for sealing against said cam ring, thereby to seal thepumping chamber, .[.means including.]. .Iadd.said housing and.Iaddend.said cheek plate forming a static chamber receiving fluid atpump-generated pressure and whereby said cheek plate is pressure-loadedagainst said cam ring, and venting means for said chamber including aventing passage and a servo valve in control of said venting passage andforming the master valve of a control system wherein the cheek plateforms a slave valve, .Iadd. means for sealing said static chamberagainst fluid flow except for said venting means and said fluidreceiving means in said static chamber including a seal between saidhousing and said cheek plate, .Iaddend. orifice means externally of saidstatic chamber in the pumping circuit of the pump for developing apressure gradient independently of the pressure in said static chamberupon buildup of pressure in the pump outlet caused by normal parasiticlosses in a downstream hydraulic flow circuit, and means for actuatingsaid servo valve as a function of the pressure drop across said orificemeans,thereby to vent said chamber to unload the cheek plate.Iadd.resulting in movement of said cheek plate relative to said housingand modulation of .Iaddend..[.and modulate.]. pump flow output.
 16. In apump as defined in claim 15, said servo valve being actuated by pressuredrop across an orifice formed in the outlet line of the pump and throughwhich fluid at pump-generated pressure is directed.
 17. In a pump asdefined in claim 15, said servo valve being actuated by pressure dropacross an orifice formed in the return line of the pump and throughwhich spent fluid is directed on the return line side of the pump. 18.In a pump as defined in claim 15, housing means and reservoir meanssuccessively outwardly adjacent said cam ring, and openings in saidhousing means forming conduits for flow from the reservoir and beingdisposed so that the fast by-pass over the cam ring from the plate liftinducts out from the reservoir to accomplish super-charge.
 19. Themethod of flow control in a pump which comprises the steps ofpressureloading a cheek plate with fluid at pump-generated pressure to form aseal between the inlet and outlet portions of the pump, directing thefluid discharged by the pump at pump-generated pressure to a point ofutilization and returning the spent fluid to the pump, sensing apressure gradient on opposite sides of an orifice, selectively ventingthe pressure acting on the cheek plate by using a servo valve, andactuating the servo valve in accordance with the changing pressure dropthrough the orifice occurring with varying pump speed, thereby to unloadthe cheek plate and allowing the outlet portions of the pump to by-passdirectly to the inlet portions thereof at the cheek plate, said methodbeing further characterized by actuating the servo valve by the pressuredrop across an orifice in the pump return line.
 20. In a pump having apressure-loaded cheek plate sealing the respective inlet and outletportions of a pumping chamber containing fluid displacement means, theimprovement ofa servo valve for venting the pressure acting on the cheekplate, means forming an orifice in the return line of the pump andthrough which spent fluid is directed on the return line side of thepump, and means for referencing the servo valve to the pressure dropacross said orifice, whereby upon loading the cheek plate the outletportions of the pump will be by-passed directly to the inlet portionsthereof.
 21. In combination,a pump having housing means forming apumping chamber having inlet and outlet portions, rotary fluiddisplacement means in said pumping chamber for moving the fluid atincreased pressure from the inlet to the outlet portions, a cheek plateadjacent said pumping chamber having a sealing face for sealing theinlet and outlet portions during operation of the pump, means includingsaid housing forming a pressure control chamber behind said cheek plateto load said cheek plate with fluid at pump-generated pressure, meansincluding said housing forming a return line for said pump receivingspent fluid returned to the pump, means forming an orifice in saidreturn line and through which spent fluid is directed on the return lineside of the pump, and a servo valve controlling a passage in saidhousing venting said pressure control chamber to a zone at lowerpressure in response to the pressure drop across said orifice,therebyunloading the cheek plate and directly by-passing the outlet portions ofthe pump to the inlet portions adjacent the sealing face of the cheekplate. .Iadd.22. In a pump having a nonrotatable axially movablepressure-loaded cheek plate sealing the respective inlet and outletportions in a pump containing rotary fluid displacement means, theimprovement of means forming a static cavity behind said cheek plate,means forming a venting passage from the static cavity to a zone at areduced pressure, pumping circuit means forming a pump outlet and a pumpreturn for placing the pump in a pumping circuit, means forming a flowcontrol orifice in said pumping circuit means, a servo valve forcontrolling said venting passage and thus venting the pressure in saidstatic cavity acting on the cheek plate, whereby the cheek plate willact as a slave and the servo valve will act as the master to establish aservo ratio effect, and means for referencing the servo valve to thepressure drop across said orifice, whereupon unloading the cheek platethe outlet portions of the pump will be by-passed directly to the inletportions thereof to modulate the pump flow output, said means forming anorifice forming said orifice in the return line of the pump and throughwhich spent fluid is directed on the return line side of the pump..Iaddend. .Iadd.23. In a pump having a nonrotatable axially movablepressure-loaded cheek plate sealing the respective inlet and outletportions in a pump containing rotary fluid displacement means, theimprovement of means forming a static cavity behind said cheek plate,means forming a venting passage from the static cavity to a zone at areduced pressure, pumping circuit means forming a pump output and a pumpreturn for placing the pump in a pumping circuit, means forming a flowcontrol orifice in said pumping circuit means, a servo valve forcontrolling said venting passage and thus venting the pressure in saidstatic cavity acting on the cheek plate, whereby the cheek plate willact as a slave and the servo valve will act as the master to establish aservo ratio effect, and means for referencing the servo valve to thepressure drop across said orifice, whereupon unloading the cheek platethe outlet portions of the pump will be by-passed directly to the inletportions thereof to modulate the pump flow output, said servo valvecomprising a spool valve having a tapered spool movable in a bore havinglands and recesses, spring means initially loading said spool in onedirection,said spool controlling the venting passage by graduallyopening the passage due to the tapered configuration, thereby to producean axial movement of the cheek plate but through a significantly lesserdistance than the corresponding movement of the spool to maintain a flowlevel corresponding to a servo ratio effect in the flow regulation ofthe pump. .Iaddend..Iadd.24. In a pump having a nonrotatable axiallymovable pressure-loaded cheek plate sealing the respective inlet andoutlet portions in a pump containing rotary fluid displacement means,the improvement ofmeans forming a static cavity behind said cheek plate,means forming a venting passage from the static cavity to a zone at areduced pressure, pumping circuit means forming a pump output and a pumpreturn for placing the pump in a pumping circuit, means forming a flowcontrol orifice in said pumping circuit means, a servo valve forcontrolling said venting passage and thus venting the pressure in saidstatic cavity acting on the cheek plate, whereby the cheek plate willact as a slave and the servo valve will act as the master to establish aservo ratio effect, means for referencing the servo valve to thepressure drop across said orifice, whereupon unloading the cheek platethe outlet portions of the pump will be by-passed directly to the inletportions thereof to modulate the pump flow output, means forming ametering passage from a source of pump generated pressure to said staticcavity, and means forming a metering orifice for said metering passage..Iaddend. .Iadd.25. A pump for supplying fluid to a system comprising,ahousing having an inlet and an outlet, said housing defining a pumpingchamber, pumping means in said chamber operable to pump fluid from saidinlet to said outlet, said pumping means including pumping elementswhich define a series of pumping pockets which expand and contract toeffect pumping of fluid, a cheek plate having one axial side adjacent toand facing said pumping means and being supported in said housing, saidcheek plate having a sealing position blocking fluid communicationbetween said pockets and being axially movable therefrom to enable fluidto flow directly between said pockets and by-pass the system, springmeans for biasing said cheek plate into said sealing position, saidhousing at least in part defining a cavity on the other axial side ofsaid cheek plate, a fluid passage for directing a flow of fluid fromsaid outlet into said cavity for urging with said spring means saidcheek plate into said sealing position, a seal between said cheek plateand said housing blocking flow into said cavity except through saidfluid passage, an orifice in said outlet and through which fluid flowsto the system, valve means for venting the fluid pressure in said cavityto enable said cheek plate to move away from said pumping means, saidvalve means including a movable valve member having surfaces acted uponby fluid pressures on opposite sides of said orifice and which ismovable to control the pressure in said cavity to control the positionof said cheek plate and thereby control the flow of fluid to the system..Iaddend..Iadd.26. A pump as defined in claim 25 wherein said valvemeans comprises a valve spool movable to vent said cavity to said inletto enable said cheek plate to move axially away from said pumping meansand wherein said orifice is defined by a surface portion carried by saidvalve spool and a surface portion carried by said housing..Iaddend..Iadd.27. A pump as defined in claim 25 wherein said valvemeans comprises a valve spool having at least two axially spaced lands,a first land cooperating with a first port communicating with saidcavity and which when moved past said first port creates a fluid flowfrom said cavity through said port past said land, and a second land forat least temporarily blocking communication between the pump outlet andpump inlet. .Iaddend..Iadd.28. A pump as defined in claim 27 whereinsaid valve spool is located in axial alignment with said orifice, andsaid valve spool has a projection extending therefrom, said orificebeing defined in part by the outer surface of said projection, saidprojection having a passage therethrough for communicating the systempressure on one side of said orifice to one portion of the valve spoolwhile outlet pressure on the other side of said orifice acts on anotherportion of said valve spool. .Iaddend. .Iadd.29. A pump as defined inclaim 28 wherein said cheek plate is nonrotatable and said fluid passagefor directing flow into said cavity comprises a passage through saidcheek plate and which passage has an orifice therein.